Revealed: how the FBI coordinated the crackdown on Occupy

[Moonwalk]

35. There is another type of fallacy, however: Argument from a questionable source...

Also known as Appeal to Authority:

You appeal to authority if you back up your reasoning by saying that it is supported by what some authority says on the subject. Most reasoning of this kind is not fallacious, and much of our knowledge properly comes from listening to authorities. However, appealing to authority as a reason to believe something is fallacious whenever the authority appealed to is not really an authority in this particular subject, when the authority cannot be trusted to tell the truth, when authorities disagree on this subject (except for the occasional lone wolf), when the reasoner misquotes the authority, and so forth. Although spotting a fallacious appeal to authority often requires some background knowledge about the subject or the authority, in brief it can be said that it is fallacious to accept the words of a supposed authority when we should be suspicious of the authority’s words.

Example:

The moon is covered with dust because the president of our neighborhood association said so.
This is a fallacious appeal to authority because, although the president is an authority on many neighborhood matters, you are given no reason to believe the president is an authority on the composition of the moon. It would be better to appeal to some astronomer or geologist. A TV commercial that gives you a testimonial from a famous film star who wears a Wilson watch and that suggests you, too, should wear that brand of watch is using a fallacious appeal to authority. The film star is an authority on how to act, not on which watch is best for you.

In short, it is not an ad hominem attack if one is questioning the authority of the source.